Metal coating for ceramic bodies



June 1, 1998. R. mms 2,119,989

IETAL COATING FOR CERAMIC BODIES Filed Aug. 13, 1937 /N VEA/TOR Haq Higgins AT TR/VEY Patented .lune 7, 1938 UNITED STATES PATENT OFFICE 18 Claim.

This invention relates to a metallic coating for ceramic ware and to a process of applying the same, and has for one of its objects the provision of a comparatively smooth metal coating for ceramic ware which will adhere firmly and withstand wear.

A further object is to provide a practical and economical method of applying the coating.

Other objects and advantages will appear from the following description.

The invention is exemplied by the form and composition of coating and the process of applying the same described in the following speciflcation and illustrated in the accompanying drawing, and it is more particularly pointed out in the appended claims.

This application is a continuation in part of my prior application, Serial No. 107,901 filed October 27, 1936.

In the drawing:

Fig. l is a part section and part elevation of a porcelain insulator having a portion of the surface thereof metallized in accordance with the present invention.

Fig. 2 is a photomicrograph of a section of porcelain magnified twenty times, the porcelain having one surface thereof metallized according to the present invention.

Fig. 3 isa photomicrograph of the surface of the base coating upon which the metal coating is disposed, also magnified twenty times.

'I'here are many instances in the industrial arts where it is desirable to apply a metal coating to ceramic ware such as porcelain insulators. Coatings of this kind are useful in the construction of electrostatic condensers, in the control of electrostatic iux, in the prevention of radio distur -I ances and in many other phases of the insulator art. They are also useful in other branches of industry and although the invention is shown and described, by way of illustration, as applied to the surfaces of a porcelain insulator, it will be understood that it is not :estricted to this use.

A number of methods have heretofore been used for applying metallic coatings to ceramic ware, such as the well known method of burning the metal onto the surface, used for many years in the decoration of china ware.

Another method of applying a metallized coating to insulators and other ceramic articles is shown in the patent to Arthur O. Austin #1,536,749, dated May 15, 1925. The coating there shown and described and the method of applying it has been widely used in connection with insulators but has some objections for certain purposes. One object of the present invention is to overcome these objections.

Difficulty has been experienced in obtaining a firm union between a metal coating and a smooth porcelain surface and in the invention disclosed in the Austin patent the porcelain surface is covered with small particles of porcelain secured to the surface by glaze and the coating is then applied to this roughened surface. It has been found in practice, however, that these particles of porcelain crush and crumble under pressure and abrasion, and also that, because of the rough nature the surface produced, conductors, in contact with this surface, are sometimes worn and abraded.

The present invention provides a surface that is suiliciently smooth to avoid all abrasion of conductors in contact with the surface and sumciently rm so that it will not crush or crumble under pressure or impact. In the present invention the metal is applied to an especially prepared surface by what is known in the trade as a Schoop gun. In this method of applying metal, a wire of the coating material is fed into an oxygen acetylene flame by which flame the metal is melted and blown upon the surface in a molten condition and with sufficient force to cause the molten metal to adhere firmly to the surface by penetrating any minute depressions and surrounding any minute projections offered by the treated surface. The metal from a Schoop gun will not, however, adhere well to an unprepared porcelain surface either glazed or unglazed. A fair degree of success may be obtained by the application of the metal to a sandblasted porcelain surface but there are dimculties inthe way of sandblasting many surfaces that it is desirable to treat.

In most cases a portion only of the surface of an insulator is metallized, and it is desirable that the portion of the surface not metallized shall be covered with a high gloss glaze. In such cases it is difficult to confine the Sandblasting process to the particular portions of the insulator to be metallized without encroaching upon the glazed surface. To avoid all these difilculties and to obtain an improved metallic coating, the present invention provides an entirely different and novel method of preparing the surface to be treated for the metal coating. This method comprises the application to the porcelain of a preliminary coating which is caused to adhere firmly to the porcelain by the nring of the piece and which presents a minutely roughened surface having very fine projections which are exceedingly strong will not crush or come loose and which are so fine that, although they enable the metal to adhere rmly to the prepared surface, they do not `-produce projections on the finished surface, as in the case of the sanded surface of the Austin patent referred to above. The preliminary coating forming the base for the coating of metal includes a bonding material which on firing will adhere firmly to the porcelain surface, and mixed With the bonding material is an aggregate of very finely ground refractory substance which will provide the necessary roughness to enable the metal to adhere firmly to the prepared surface. The bonding material and aggregate are mixed with a suitable suspension medium to facilitate the application of the base coating and to cause lt to adhere evenly to the porcelain surface prior to firing. Any suitable flux or glazing material that will form a firm bond with the porcelain on firing may be used for the bonding material. I have found a metal oxide, such as ferric oxide, well adapted for this purpose.

The aggregate may consist of any one of a considerable number of highly refractory substances, such as andalusite, kyanite, mullite, silicates of zirconium, rutile, alundum, carborundum, corundum and sillimanite. I have found that a mixture of sillimanite and an electric furnace, brous mullite, which form of mullite is known by the trade name of Durox and is produced by the Vitrefrax Corporation of Los Angeles, California, provide a very satisfactory aggregate for the base coating. I prefer the ceramic grade of Durox which is a well known commercial grade of Durox sold to the trade for ceramic use and its composition and method of manufacture are more fully,

explained in a paper by Thomas S. Curtis published in vol. 11, No. 12, of December, 1928 of the Journal of the American Ceramic Society.

One very satisfactory coating mixture may comprise the following:

Percent Ferrie oxide 20 Durox (ceramic grade) 24 Sillimante 3 Liquid glue 1/2 Glycerine 34 Distilled water 181/2 The Durox is preferably ground to pass through an 80 mesh screen and the sillimanite through a 100 mesh screen. One advantage in grinding the aggregate so fine is that the resulting projections and depressions on the surface of the coating, although sufficient to provide a firm anchorage for the metal, can easily be covered by a thin coating such as can be applied by a spray gun,

so as to even up the inequalities and leave a smooth outer surface of metal. If large grains of aggregate are used such as shown in the prior Austin patent referred to above, a coating of metal sufficiently thick to bury the grains and produce a smooth outer surface is too apt to peel off. Then too, where large grains of aggregate are used, any difference in expansion or contraction between the porcelain body and the grains of aggregate will cause the grains to separate from the porcelain, but where the grains are as small as those employed by applicant, this effect is not present. Then too, the fine grains may be held in suspension long enough to apply to the porcelain surface much more readily than coarse grains.

The glue may be a good grade of nsh glue and this with the glycerine provides an adhesive, slightly viscous liquid, which enables the operator to apply the material to the porcelain in a thin even coating which remains in place prior to firing.

The ferrie oxide should be finely ground with a portion of the liquids for four to live hours in a suitable pebble mill. It has been found that improved results may be obtained by replacing one-half of the commercial ferric oxide with a residue obtained by dissolving steel filings in dilute nitric acid.

A thinner for the above coating may consist of a mixture of red oxide of iron- 15 ounces; residue from iron dissolved in nitric acid-l5 ounces; Le Pages liquid glue-2-ounces; glycerine-60 ounces; distilled water- 24 ounces. Normal operation will require about one part of the thinner to twenty parts of the base coating material. It will be necessary to supply a little thinner from time to time to the glaze in the working container, but only enough should be used to produce a free flowing consistency. Too much thinner will result in a deficiency of the fine grains of aggregate. The success of the base coating depends upon the uniform distribution of the fine grains of aggregate contained in the base coating. The particles are heavy and settle rapidly so that care must be exercised to keep the mixture well stirred. In applying the base coat, all other glaze and sanding operations should be performed before the application of the coating material. The body glaze should be run as close as possible to the zone to be metallized and the base coating should then be applied so as to overlap slightly the body glaze. The coating is applied as uniformly as possible by a slow motion ofthe brush, only one coating being applied. It may also be applied by means of a spray gun.

One important characteristic of the coating is that in firing it matures with the porcelain and the correct firing of the porcelain is also correct for the coating. Because of this property it is not necessary for the kiln operator to make any special provision for the coating, but he can nre the porcelain in the usual way and the coating will be correctly fired. The usual temperature used is from ()a to 1285" C. and the correct temperature and time of firing is indicated by collapse of pyrometric cones nine to eleven. Since the coating matures with the porcelain, the process is a single flre process making lt much more economical and convenient than prior coating methods requiring two or more res. The particles of Durox and sillirnanite aggregate are harder, stronger and more refractory than porcelain and, after firingf will still retain their separate identities and provide the necessary minute projections and depressions for the attachment of the metal coating. However, due to the presence of flux in the binding material, the particles, during firing, will fuse slightly on the outside, sufficiently to cause them to adhere firmly to the porcelain but not sufciently to destroy the separate identities of the particles. It has been found that better results are obtained if the ware is fired in an oxidizing or at least a non-reducing atmosphere. The success of the coating is largely due to the strength of the minute grains of aggregate. If insufliciently fired, the coating will not be strong enough, and if overiired, the metal will not adhere well to the surface so that it is a simple matter for the operator to tell when the nring has been properly done or to correct any error in this respect.

In the application of the metal, the blast should be directed squarely against the face of the surface so that the particles of metal will be driven into intimate contact with the base coating and forced into the crevasses and around the projections. By this means it has been found that a metal coating of copper can be applied to a porcelain surface so firmly that it will withstand blows and abrasion and will break of! particles of the porcelain rather than peel from the surface.

In Fig. l of the drawing is illustrated an insulator comprising a porcelain body Il having its upper surface and tie wire groove provided with a coating i I applied according to the present invention. 'Ihis coating affords intimate electrical connection between the conductor or tie wire and the surface of the insulator so as to avoid formation of corona and radio disturbances. The coated surface also provides an extended area for distribution of the electrostatic flux between the supporting pin il and the conductor so that the flux is directed largely through the porcelain, preventing concentration in the air space which would give rise to the formation of corona. This, of course, illustrates only one application of the invention, of which there are many others which will readily suggest themselves to those skilled in the art.

Fig. 2 is a section through a porcelain body having a base coating applied thereto and a coating of copper applied over the base coating. 'I'he base coating is shown by the relatively dark layer- I4 and the copper by the lighter layer I3 overlying the darker layer. It will be seen that the copper fills all the minute crevasses and surrounds the projections on the surface of the base coating so that it adheres firmly to the prepared surface. In practice, it has been found that the bond between the copper and the base coating and between the base coating and the porcelain is even stronger than the porcelain body.

Fig. 3 is a magnified view after firing of a surface treated with the base coating applied according to the present invention. The illustration is a copy of a photomicrograph magnified twenty times so as to show the minute projections and depressions which provide an anchorage for the metal to be applied by the Bchoop gun.

I claim:

1. The method of metallizing the surface of a ceramic body comprising the steps of applying to said surface, a base coating comprising a suitable 5g bonding material and a fine aggregate of highly refractory material of greater strength than the material of said body, firing the body and coating and thereafter applying a coating of metal over the surface of said base coating, the particles of said aggregate being sufficiently refractory to retain their separate identities at the ring temperature for said ceramic body so as to produce a rough surface to receive said metal coating.

2. The process of metallizing the surface of a ceramic body comprising the steps of applying to said surface, a base coating comprising a suitable bonding material, and an aggregate of highly refractory material ground sufllciently une to pass through an 80 mesh screen, simultaneously firing said body and base coating and applying a coating of metal to said base coating, the particles of said aggregate. being sufllciently refractory to retain their individual identities at the firing temperature for said ceramic body.

3. The method of metallizing th surface of a ceramic body comprising the stops of applying to said surface a base coating including a bonding material and an aggregate of finely ground material which is more refractory than the material of said body, firing said body with said base coating thereon sufficiently to vitrify said bonding material and thereby fix said aggregate to said body, but not sufdciently to destroy the separate identities of the particles of aggregate and applying a metal coating to the base coating after firing, of sufficient thickness to ll the spaces between said particles to produce a smooth outer surface.

4. The method of metallizing the surface of a ceramic body comprising the steps of applying and firing to said surface a base coating having projections and depressions on the surface thereof of suitable size and spacing to retain a metal coating applied thereto and applying a metal coating over said base coating to a depth greater than the maximum height of said projections over the bases of said depressions and thus providing an outer surface on which the projections and depressions of said base coating are obliterated.

5. The method of metallizing the surface of a ceramic body comprising the steps of applying a base coating of refractory material to said body, simultaneously firing said body and base coating said base coating, after firing having projections and depressions on the surface thereof of suitable size and spacing to form an anchorage for a coating of metal and applying a metal coating to said base coating, said metal coating filling said depressions and surrounding said projections and 'being of sufiicient thickness to bury the projections on the base coating so that said projections do not affect the contour of the outer surface of the metal coating.

6. The method of metallizingv the surface of a ceramic body comprising the steps of applying a base coating, including a fluxing material, a suspension medium and an aggregate of finely ground, highly refractory material, the particles of which aggregate are fine enough to pass through an 80 mesh screen, firing said ceramic body and base coating sufllciently to fuse the outer surfaces of the particles of said aggregate in the presence of said iiuxirig material'without destroying the separate .identity of said particles so that said particles provide an irregular anchorage surface firmly bonded to said body and thereafter applying a ,metal coating to said anchorage surface.

7. The method of metallizing the surface of a ceramic body comprising the steps of applying to said body a base coating including a iiuxing material, a suspension "medium and an aggregate of finely ground, highly refractory material, said refractory material belonging to the group composed of andalusite, kyanite, mullite, silicates of zirconium, rutile, alundum, carborundum, corundum and sillimanite, firing said body and base coating sufiiciently to bond the aggregate to the body but not sufnciently to destroy the separate identities of the particles of aggregate and thereafter applying to the surface of said base coating a coating of metal.

s. The method of memiuzing the surfacev rfa'" ceramic body comprising the steps of applyingv to said surface s .basa coating. including a fiuxing material, and an `aggregate gf finely ground mullite, firing said body and basecoating yaufliciently to bond said aggregate to said body Without destroying the individual identities of the surface of said. base'coating a coating of metal. 9. The method of metallizing the surface of a ceramic body comprising the steps of applying tothe surface of said body before firing a base coating, including a iluxing material, and finely ground Durox, firing said body and 'base' coating sufficiently to bond the aggregate to said body but not sufliciently to destroy the separate identities of the particles of aggregate and thereafter applying to the surface of said basecoating a coating of metal.

10` The method of metallizing the surface of a ceramic body comprising the steps of applying to said surface before firing a base coating including a fiuxing material, and an aggregate of finely ground mullite and finely ground sillimanite, firing said body and base coatingl sumciently to bond the aggregate to said body but not sufficiently to destroy the separate identities of the particles of aggregate and thereafter applying to the surface of said base coating a coating of metal.

l1. The method of metallizing the surface of a ceramic body comprising the steps of applying to said surface a base coating, including a fluxing material, a suspension medium and an aggregate, said aggregate including mulli'te ground to a flneness to pass through an 80 mesh screen, and sillimanite ground to a fineness to pass through a 100 mesh screen, firing said body and base coating sufficiently to bond the aggregate to said body but not sufdciently to destroy the separate identities of the particles of aggregate and thereafter applying to the surface of said base coating a coating of metal.

12. The method of metalliaing the surface of a ceramic body comprising the steps of applying to said surface a base coating, including a uxing material and an aggregate of finely ground, highly refractory material, the grains of said refractory material differing in size from one another,

`firing said body and base coating sufficiently to bond said aggregate to said body but not sufficiently to destroy the separate identities of the particles of aggregate and thereafter applying to the surface oi' said base coating a coating of metal of sufficient thickness to iill all of the depressions in the surface of said base coating and cover all of the projections thereon.

13. The combination 'with a ceramic body, of

14. The combination with a ceramic body, of

a surface coating thereon comprising a base layer of finely ground aggregate of greater strength than the material of the body, said particl of aggregate being sufficiently refractory to retain theirindividual identities atthe firing temperature for said ceramic body and bonded to said body and forming irregularities on the surface thereof to provide an anchorage, and a coating of metal over said base layer and filling the depressions and overlying the projections formed by the particles of said aggregate.

15. 'I'he combination with a ceramic body, of

a surface coating thereon comprising a base layer of finely ground aggregate formed of material included in the group consisting of andalusite, kyanite, mullite, silicates of zirconium, rutile, alundum, carborundum, corundum and sillimanite, said aggregate being bonded to the surfaces of said body, and a coating of metal overlying said base layer and filling the depressions and overlying the projections formed by said aggregate. 16. The combination with a ceramic body, of -a surface coating for said body comprising a base layer of finely ground aggregate formed of a mixture of ceramic grade of Durox and sillimanite, said Durox being ground to pass through an mesh screen and said sillimanite being ground to pass through a 100 mesh screen, the particles of said aggregate being bonded to the surface of said ceramic body, and a metal coating on the surface of said base layer and anchored thereto by the depressions and projections formed by said aggregate, the thickness of said metal coating being greater than the maximum dimensions of the particles of said aggre gate and filling the depressions and overlying the projections formed by said particles.

17. The method of treating the surface of a ceramic body comprising the steps of applying to said body before firing a base coating including a fiuxing material, a suspension medium and an aggregate of finely ground refractory material, said refractory material belonging to the group composed of andalusite, kyanite, mullite, silicates of zirconium, rutile, alundum, carborundum, corundum and sillimanite, and firing said body and base coating sufficiently to bond said aggregate to said surface but not sufficiently to destroy the separate identities of the particles of aggregate. y

18. The combination with a ceramic body, of a coating on the surface of said body comprising `alayer of fine aggregate of highly refractory material belonging to the group composed of andalusite, kyanite, mullite, silicates of zirconium, rutile, alundum, carborundum, corundum' and sillimanite, the particles of said aggregate being fused to said body but retaining separate identities.

-RAY HIGGINS. 

